The present invention relates to a braking-force control system for a road vehicle having an electrically controllable four-circuit brake system, in which the braking forces which can be generated by the individual wheel brakes can be adjusted in a wheel-specific way. A chassis is equipped with adjustment devices for varying the normal forces acting on the vehicle wheels, at least one normal-force adjustment device being provided per vehicle wheel. The brake system is configured so that the braking force, which can be generated by a front-wheel brake and the rear-wheel brake which lies opposite the latter viewed along the diagonal of the vehicle, corresponds overall to the vehicle weight multiplied by a maximum coefficient of friction which is effective between the carriageway and the braked wheels. The adjustment devices for the distribution of normal force are configured so that, for the most part, i.e. with the exception of small fractions, in the range of a few per cent (for example 5%) of the wheel loads which are symmetrically and approximately uniformly distributed in a static case, the weight of the vehicle can be supported on one of the front wheels and on the rear wheel of the vehicle which is arranged diagonally opposite the latter.
The braking-force control system according to the invention provides at least the following advantages:
By means of the normal-force adjustment devices alone, effective vehicle-movement regulation in order to compensate oversteering or understeering behavior of the vehicle is already possible without a vehicle-movement regulation system which operates with the build-up of brake slip being necessary from the outset. The regulation system instead only needs to be activated if the control of the distribution of the normal force is no longer sufficient to achieve a desired cornering behavior. As a result, brake wear is reduced. The two wheel brakes of one vehicle diagonal achieve virtually the same vehicle deceleration which it would otherwise only be possible to achieve by activating all the wheel brakes. Accordingly, when one wheel brake fails or when both wheel brakes which lie opposite one another along a diagonal of the vehicle fail, virtually the maximum deceleration of the vehicle can still be achieved, which can be considered a considerable safety benefit.
DE 42 01 496 A1 shows that the changes in wheel load which result from the rolling moments occurring during cornering by actuating hydraulic chassis adjustment devices can be distributed in such a way that a virtually neutral steering behavior which is aimed at is obtained. DE 40 14 561 also teaches that pitching moments, which occur as a result of controlled changing of the resilience of suspension or shock-absorber elements, can be absorbed which also corresponds to a change in the distribution of the wheel load.
In contrast to the known control devices in which the distribution of the wheel load can only be changed within relatively narrow limits, i.e. in relation to the absolute value of the rolling and pitching moments which occur, in the braking-force control system according to the invention a maximum possible variation of the distribution of the wheel load is provided in order to achieve a correspondingly effective expansion of the vehicle-movement regulating range which is necessary to enable a vehicle-movement brake intervention to be utilized to the greatest possible degree.
In an advantageous embodiment of the braking-force control system, the increase in the braking force which is necessary for a "two-wheel braking operation" by just one front-wheel brake and the rear-wheel brake which is diagonally opposite is achieved by pressure-increasing actuation of the brake-pressure adjustment devices which, because this method of braking is intended as it were only for an emergency, is readily possible and does not lead to appreciable additional wear.
At the same time, it may be expedient if the brakes have, for example, four-piston brake callipers, just one pair of pistons being utilized in normal braking mode, and the further pairs of pistons being utilized for generating braking force in diagonal braking mode.
It is particularly advantageous if the braking-force control system is configured to the effect that, for reverse travel, it is possible to use a braking-force distribution with a greater proportion of braking force at the rear axle than the proportion of braking force at the front axle so that even when reversing under poor road conditions the best possible decelerations of the vehicle can still be achieved.
If only one of the wheel brakes is used for generating braking force at the start of a braking operation, conclusions regarding the condition of the brake linings can be drawn from a comparison of a measured value of the vehicle deceleration with a value of the setpoint deceleration which has been calculated on the basis of the geometric brake data and the measured brake pressure, and a diagnosis of this can easily be carried out.
In an analogous way, it is possible to determine the current value of the coefficient of friction (.mu.) of the carriageway by activating only one of the wheel brakes at certain times. Such activation operations of just one wheel brake are expediently carried out at regular time intervals and in driving situations in which the driver does not activate the brake system and the vehicle rolls with an essentially constant speed. In such braking cycles which are carried out in order to detect the coefficient of friction, the braked wheel is releived of loading by activating its normal-force adjustment device to such an extent that the wheel load of the braked wheel is reduced to a small fraction of, for exampl, just 10% to 30% of its wheel load which is effective during unbraked travelling so that the braking of this wheel results only in a decleration of the vehicle which cannot be sensed by the driver. Such decleration can also be compensated by virtue of the fact that a compensatory increase in the propulsion moment generated by the vehicle engine is automatically applied during the measurement cycle of the coefficient of friction, so that the reaction on the vehicle which is produced as a result of the individual-wheel braking operation is virtually compensated. During such a braking operation, the wheel-braking force which is applied is increased unitl the brake slip occuring at this wheel reaches a value of, for example, 30% to 405 which is characteristic of a maximum possible utilization of adhesion.
In a preferred embodiment of the braking-force control system, even in the case of partial or full braking which is initiated by the driver and is intended to bring about significant deceleration of the vehicle (and, if appropriate, also a braking operation which is directed, at least in its initial phase, as an automatically controlled full braking (brake assistant function)), the coefficient of friction .mu. of the carriageway which can be utilized is determined. For this purpose, the braking-force and the normal-force control systems are actuated so that, at the start of the braking operation, an overwhelming proportion of the vehicle weight is supported by just two vehicle wheels which lie diagonally opposite one another and which are then appropriately braked to a greater extent, and the wheels of the other diagonal of the vehicle are correspondingly relieved of loading. At the vehicle wheels which have been relieved of loading, the wheel brakes are actuated with an incremental or monotonously continuous increase in the brake pressure, and the rate of change d.omega./dt of the wheel speed is continuously determined. For the brake pressure p.sub.B at which the maximum rate of change of the wheel speed occurs, the maximum coefficient of friction .mu. of the carriageway which can be utilized is determined according to the relation ##EQU1##
in which the moment of inertia of the braked wheel which has been relieved of loading is designated by .THETA., the wheel load which is acting on this wheel is designated by F.sub.N, its dynamic rolling radius is designated by r.sub.dyn, the braking torque which is given by the setting of the brake pressure at the maximum change of rate of the wheel speed is designated by M.sub.B, and the braking torque M.sub.B itself is given by the relation EQU M.sub.B =p.sub.B.multidot.C.sub.B
in which the applied brake pressure is designated by p.sub.B, and a proportionality factor which takes into account the configuration of the brakes is designated by C.sub.B.
This determination of the coefficient of friction of the carriageway which can be utilized, both for the left-hand and for the right-hand side of the vehicle, also makes it possible to detect, as it were, in one measurement cycle, whether these coefficients of friction are different from one another, i.e. ".mu.-split conditions" apply which are particularly critical, for example when regulating brake slip, so that awareness of them is particularly important, as it were, for preselecting regulating parameters.
A variant of a braking-force control system according to the invention has the advantage that, independently of the load state of the vehicle and of the topography of the carriageway, the same, defined value of the deceleration of the vehicle is always associated with a defined pedal position of a braking-force setpoint value signal transmitter, as a result of which the braking comfort is considerably improved.
In the case of a vehicle which has at least one electric drive or an electric auxiliary drive which, in order to generate braking force, can be switched over into a recuperation mode which is suitable for charging the vehicle battery, one configuration of the braking-force control system is particularly advantageous to the effect that although the recuperation capability is used to the greatest possible extent to generate braking force, accompanied by permanent monitoring of the operating and functional status of the vehicle battery, the hydraulic wheel brakes are also activated at the correct time before rapid charging of the vehicle battery is no longer possible and there could be a drop in the deceleration of the vehicle due to the reduction of the recuperation braking torque, so that a setpoint value-controlled vehicle deceleration can reliably be maintained. This demand-adjusted utilization of the power which can be recuperated electrically in order to generate braking force makes it possible both to operate the hydraulic brakes in a wear-minimizing way and also permits optimum use of the capacity of a battery without overloading the battery.
With a brake-force control system according to the present invention, it is also possible to implement an effective regulating operation which is aimed at stable vehicle-movement behavior of a road vehicle and in whose initial phase a vehicle-movement regulating operation is carried out by tensioning the chassis by way of the normal-force adjustment devices to bring about the respectively most favorable distribution of the wheel load in terms of vehicle movement, and if it is still not possible to achieve vehicle-movement stability to a sufficient degree using this approach alone, the distribution of cornering force between the front wheels and the rear wheels of the vehicle in a way which is most favorable in terms of vehicle movement is achieved by additional braking intervention and resulting build-up of brake slip.